The present invention relates to a lithographic printing plate having an aluminum sheet as a support and particularly to a lithographic printing plate subjected to silver complex diffusion transfer process.
As for the lithographic printing plates made using the silver complex diffusion transfer process (DTR process), some examples are described in Andre Rot and Edis Wide, "Photographic Silver Halide Diffusion Process", pages 101-130, published from Focal Press, London and New York (1972).
As described therein, lithographic printing plates made using DTR process include two types, namely, the two-sheet type in which a transfer material and an image receiving material are separated and mono-sheet type which comprises the transfer material and the image receiving material which are provided on one support. The two-sheet type lithographic printing plates are described in detail in Japanese Patent Kokai No. 57-158844. The mono-sheet type lithographic printing plates are described in detail in Japanese Patent Kokoku Nos. 48-30562 and 51-15765 and Japanese Patent Kokai Nos. 51-111103 and 52-150105.
In the case of lithographic printing plates using paper as a support, it is difficult to obtain prints of high qualities including printing endurance owing to the elongation of plate or permeation of water during printing. In order to improve these problems and enhance printing performances, film supports are used. Examples of the films utilized as a support are cellulose acetate film, polyvinyl acetal film, polystyrene film, polypropylene film, polyethylene terephthalate film, and composite films such as polyester, polypropylene or polystyrene films clad with a polyethylene film.
The lithographic printing plates using a film as a support are improved in elongation of plate or permeation of water as compared with the printing plates using the paper support, but they are still insufficient in printing endurance, water retention and mountability on printing machines.
In order to solve the various problems of the lithographic printing plates in which paper or film supports are used, silver salt type lithographic printing plates in which metallic supports, especially aluminum supports are used are known and disclosed in Japanese Patent Kokai Nos. 57-118244, 57-158844, 63-260491, 3-116151 and 4-282295.
Japanese Patent Kokai Nos. 63-260491, 3-116151 and 4-282295 describe lithographic printing plate materials of mono-sheet type which comprise a grained and anodized aluminum sheet as a support, physical development nuclei provided on the support and a photosensitive silver halide emulsion layer provided on the nuclei and to which the DTR process is applied. The lithographic printing plate materials are exposed imagewise and subjected to DTR development and then, the silver halide emulsion layer is removed by washing with warm water to make printing plates. However, these patent publications concern with techniques directed to prevention of corrosion of the aluminum sheet caused by contact of the aluminum sheet with the silver halide emulsion and the printing plates are not satisfactory in ink-receptivity of transferred silver images and in printing endurance.
In general, the aluminum sheet used for lithographic printing plate materials is grained in its surface for improving wettability (water retention) by dampening water in printing and furthermore, for improving adhesion to the photosensitive layer. The graining can be carried out mechanically by, for example, ball, wire and brush graining, chemically by dissolving aluminum with chlorides, fluorides, etc., and electrolytically by dissolving aluminum, and combinations of these treatments. See, for example, Japanese Patent Kokai Nos. 48-28123, 53-123204, 54-146234, 55-25381, 55-132294, 56-55291, 56-150593, 56-28893, and 58-167196, and U.S. Pat. Nos. 2,344,510, 3,861,917 and 4,301,229. With reference to the surface configuration of the aluminum sheet, U.S. Pat. No. 2,344,510 describes grain structure made by carrying out mechanical graining and electrolytic graining, U.S. Pat. No. 4,301,229 describes cumulative frequency distribution of pit diameter and arithmetical mean deviation of profile, U.S. Pat. No. 3,861,917 describes depth of grained surface, Canadian Patent No. 955449 describes height and diameter of peaks of grained surface and West German Patent No. 1,813,443 describes difference of altitude of grained surface.
After subjected to these graining treatments, the aluminum sheet is subjected to anodizing treatment in an electrolyte such as sulfuric acid, phosphoric acid, nitric acid or a mixed solution thereof.
As mentioned above, there have been proposed various aluminum sheets for printing plates which differ in surface configuration and structure and which are produced under various surface treating conditions depending on kinds of lithographic printing plates and uses of printing. However, in the case of the lithographic printing plate to which the present invention is applied, except the functions to improve wettability with dampening water (water retention) and to improve adhesion to the photosensitive layer, it has not been known that the diffusion transfer development of silver halide affects the printing function of silver film formed on the surface of the aluminum sheet, specifically, it has not been known which surface configuration of aluminum sheets is preferred for ink-receptivity and printing endurance. Furthermore, no elucidation has been made on the factors for forming silver images of high quality and high printing endurance on the surface of the aluminum sheet by controlling the diffusion transfer development of the silver halide.
Especially, the direct plate-making method has begun to be put to practical use which comprises subjecting printing plate materials to flash exposure of very high intensity and short time (10.sup.-5 second or less) with various laser beams such as helium-neon laser, argon laser, and semiconductor laser, light emitting diode (LED), cathode ray tube (CRT) by scanning exposure method, and the printing plate materials which use an aluminum sheet as a support are also strongly demanded to have characteristics and performances suitable to the above exposing methods.
U.S. Pat. No. 4,621,041 discloses that good printing characteristics with high sensitivity can be given to DTR printing plate materials having a physical development nuclei layer as a surface layer which are to be subjected to scanning type exposure by using a silver halide emulsion containing at least 70 mol % of silver chloride and a water-soluble gold compound and to which is added a water-soluble iridium compound at the time of emulsification or physical ripening. However, there is the problem that printing plates excellent in quality and printing endurance cannot necessarily be obtained by mere use of the silver halide emulsion described in U.S. Pat. No. 4,621,041 as a silver halide emulsion of lithographic printing plate materials having physical development nuclei between an aluminum support and a silver halide emulsion layer.
On the other hand, negative.positive lithographic printing plate materials of high sensitivity are needed in the system of direct outputting to printing plate materials from microfilms. Furthermore, with recent development of electronics, an attempt has been made to carry out direct outputting to printing plate materials through an image setter without using an intermediate pre-press film by combination processing of letters and images. The image setters used in this case use scanning type exposing light sources such as argon laser, helium-neon laser, red light emitting diode (red LED), red semiconductor laser (red LD), and infrared semiconductor laser (infrared LD). In the case of lithographic printing plate materials described in Japanese Patent Kokai Nos.63-260491, 3-116151 and 4-282295 which are of mono-sheet type comprising a grained and anodized aluminum support, physical development nuclei provided on the support and a photosensitive silver halide emulsion layer provided on the physical development nuclei and which utilize the DTR process, usually the exposed portion is non-image portion when the above image setters are used and image reversing outputting must be conducted. Moreover, for some apparatuses, for example, an external drum, there may be the portions which cannot be exposed, such as the portions held on the drum. Therefore, preferred are negative type lithographic printing plate materials in which the exposed portion is image portion. Although there is the description in Japanese Patent Kokai No. 3-116151 that such negative type lithographic printing plate materials are useful for laser exposure, no preferable specific embodiments have been elucidated.
The inventors have conducted various investigations on the processing method of the above-mentioned lithographic printing plate materials which use previously fogged direct positive silver halide emulsions and have found the following problems. That is, when these lithographic printing plate materials are developed with the developers described in the above patent publications, the resulting images are inferior in quality and the resulting printing plates are insufficient in printing endurance.